1. Field of the Invention
The present invention relates to a lithographic apparatus and a method of cleaning a collector included in such a lithographic apparatus.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In some current designs for lithographic apparatus, radiation in the EUV range (with wavelengths typically of 5-20 nm) is used to transfer a pattern from a mask onto a substrate. Radiation produced by an EUV light source is collected by a EUV collector and then directed into an illumination system. When an EUV source based on Sn is used, it will also produce Sn particles that may contaminate the EUV collector. In order to achieve sufficient lifetime for the lithography apparatus, it is required that these Sn particles are removed from the EUV collector mirror (this will be referred to as ‘cleaning’).
Currently there are two methods available for removing these Sn contaminations. In recent experiments, it has been demonstrated that with hydrogen radicals (or a mixture of hydrogen with another gas) it is possible to remove Sn contaminations from a substrate. Another possible method is halogen cleaning. In this method halogens form a volatile Sn-halogen molecule, which evaporates and is transported out of the collector with a gas flow. Besides the Sn contamination, the collector may be contaminated with Sn-oxides. In order to remove the Sn-oxides as well, they first need to be reduced to Sn. This is done using hydrogen radicals.
Both these cleaning methods require hydrogen radicals and therefore implementation of hydrogen radicals in the EUV collector is very important. One of the requirements for efficiently cleaning a collector surface is a sufficiently large flux of hydrogen radicals on the collector surface. A large flux may be achieved at a relatively high pressure but at very high pressures hydrogen radicals are quickly lost due to three-body recombination. At low pressures the radicals are quickly lost due to recombination on wall surfaces. A possible pressure range wherein a sufficiently high flux can be established and wherein recombination of radicals is limited, is at pressures above 10 kPa. At these pressures, a high velocity flow through the collector is needed in order to clean the whole collector. However, the velocity achievable is limited by the maximum pumping capacity of the system.